The present invention relates to an overcurrent protection circuit for electrical systems, such as power supplies, and in particular, an off-line high efficiency linear power supply.
In the above-identified U.S. patent application Ser. No. 10/458,608, a high efficiency off-line linear power supply is described. The power supply is designed to provide current to electronic circuits during a time period when a dimmer circuit for electric lamps is not drawing current. In particular, the power supply is designed to draw current for powering electronic circuits during time periods when the triac of the dimmer circuit is off.
With reference to FIG. 2, this figure shows an AC waveform (dashed line) along with a power supply current draw waveform (solid line). When the dimmer is off, the triac is off for the full length of the half-cycle. In this case, it is during the periods 1 and 3 that the power supply of the above-identified patent application provides power to a storage capacitor, which is subsequently regulated by a linear regulator. The power supply does not draw current during period 2. Because of the distinctive “cat-ear” regions 1 and 3 during which the power supply provides charging current to the storage capacitor, it is sometimes referred to as a “cat-ear” power supply. When the dimmer is set at full intensity or some intermediate level between 0% and 100%, the triac is off for some portion of each half-cycle and on for the other portion of the half cycle. Now, the power supply provides power to the storage capacitor only during period 1 and does not draw current during periods 2 and 3. In both cases above, the power supply draws current when the triac is off and there is voltage available across the triac to charge the storage capacitor. Since the dimmer will never be off all the time, the power supply preferably only draws current during period 1 in all cases.
With reference to FIG. 1, this figure shows a power supply similar to the high efficiency off-line linear power supply disclosed in the above-identified copending U.S. patent application. Power is applied from an alternating current source at the input I which is rectified by a diode D1 to provide a half-wave rectified voltage level on bus V+. Alternatively, a full-wave rectified voltage from a full-wave bridge can be provided to bus V+. A power switching transistor Q1 is provided in series with the bus. The source of the transistor Q1 is provided to an unregulated voltage bus capacitor C4. Regulator U1 supplies a regulated output voltage Vo.
The power supply includes a gate voltage supply including resistors R1, diode D2, capacitor C1, and zener diode Z1, which operate essentially the way as described in the above-identified copending patent application to provide a hard gate voltage turn-on for transistor Q1 via resistor R3, diode D3 and resistor R5. The voltage provided to the gate of transistor Q1 by this circuit provides a hard turn-on of transistor Q1, reducing the power loss in transistor Q1 when transistor Q1 is on.
Transistor Q2 turns off the transistor Q1 when the voltage level at its base, as defined by a voltage divider comprising resistors R1 and R2, reaches the threshold to turn on transistor Q2. This occurs when the bus voltage on bus V+ exceeds a predefined value, typically when the triac of the associated dimmer turns on and the bus V+ waveform is in region 2 of FIG. 2. Transistor Q2 can also be turned on when the voltage on capacitor C4 exceeds a predetermined value set by Z2. When transistor Q2 turns on, the gate drive is removed to transistor Q1 and transistor Q1 is turned off. When transistor Q2 turns off, for example, in region 3, transistor Q1 is switched back on.
The circuit of FIG. 1 includes an overcurrent protection circuit 100. That circuit includes a transistor Q3 and a resistor R6 of low resistance in series with the transistor Q1. The resistor R6 passes the full load current and accordingly, results in a power loss on the order of approximately 0.9 watt for current levels of approximately 3 amps. The overcurrent protection circuit 100 operates such that if the current level through transistor Q1 exceeds the predetermined value, transistor Q3 is turned on, thereby turning off the gate drive to the transistor Q1 and preventing damage to the transistor Q1.
Overcurrent protection circuit 100 of the power supply circuit of FIG. 1 wastes power in the series resistor R6 and contributes to an unnecessary voltage drop to the unregulated bus.
It is desirable to provide an overcurrent protection circuit that results in less power loss but still adequately protects the power switching transistor.